THE PHYLOGENETICALLY CONSERVED HISTIDINES OF ESCHERICHIA-COLI PORPHOBILINOGEN SYNTHASE ARE NOT REQUIRED FOR CATALYSIS

Porphobilinogen synthase (PEGS) is a metalloenzyme that catalyzes the first common step of tetrapyrrole biosynthesis, the asymmetric condens ation of two molecules of 5-aminolevulinic acid (ALA) to form porphobi linogen. Chemical modification data implicate histidine as a catalytic residue of PBGS from both plants and mammals. Histidine may participa te in the abstraction of two non-ionizable protons from each substrate molecule at the active site. Only one histidine is species-invariant among 17 known sequences of PBGS which have high overall sequence simi larity. In Escherichia coli PBGS, this histidine is His(128). We perfo rmed site-directed mutagenesis on His(128), replacing it with alanine. The mutant protein H128A is catalytically active. His(128) is part of a histidine- and cysteine-rich region of the sequence that is implica ted in metal binding. The apparent K-d for Zn(II) binding to H128A is about an order of magnitude higher than for the wild type protein. E. coli PBGS also contains His(126) which is conserved through the mammal ian, fungal, and some bacterial PBGS. We mutated His(126) to, alanine, and both His(126) and His(128) simultaneously to alanine, All mutant proteins are catalytically competent; the V-max values for H128A (44 u nits/mg), H126A (75 units/mg), and H126/128A (61 units/mg) were simila r to wild type PBGS (50 units/mg) in the presence of saturating concen trations of metal ions. The apparent K-d for Zn(II) of H126A and H126/ 128A is not appreciably different from wild type. The activity of wild type and mutant proteins are all stimulated by an allosteric Mg(II); the mutant proteins all have a reduced affinity for Mg(II). We observe a pK(alpha) of similar to 7.5 in the wild type PBGS k(cat)/K-m pH pro file as well as in those of H128A and H126/128A, suggesting that this pK(alpha) is not the result of protonatioaldeprotonation of one of the se histidines. H128A and H126/128A have a significantly increased K-m value for the substrate ALA. This is consistent with a role for one or both of these histidines as a ligand to the required Zn(II) of E. col i PBGS, which is known to participate in substrate binding. Past chemi cal modification may have inactivated the PBGS by blocking Zn(II) and ALA binding. In addition, the decreased K-m for E. coli PBGS at basic pH allows for the quantitation of active sites at four per octamer.